Aid! I have a couple of doubts with frontend and I am a beginner

Hello programmer/developer friends (maybe for some the answer to my next question is a bit obvious, but I am a beginner in front-end development).

Recently I was practicing ❝Link to relative page❞ in HTML to link some data like ❝contact❞ or ❝about us❞ when clicking on a text), and I came across the following questions:

When making a new folder to empty the corresponding information, do I have to apply CSS and JavaScript as if it were the main folder?

Would that procedure be done with each and every additional folder I create?

Finally, will it be necessary for each of these folders to have a Back-End or would the Back-End already be something generalized?

I hope you can answer me, I hope I have explained myself well (PS: My native language is not English, so I am using a translator) ;D

I am trying to expand my knowledge a little more and remove doubts, I hope you can help me on my path as a developer.